201126565 六、發明說明： 【發明所屬之技術領域】 本發明係關於放電燈。尤其是關於於陰極將钍（Th ) 作爲射極而使用之放電燈。 【先前技術】 先前，於液晶或半導體的曝光裝置，作爲光源，使用 高壓水銀燈，又，於映像機的光源，使用氙燈（xenon lamp)。該等放電燈係被要求點燈中電弧安定（電弧安定 性）及可長時間維持一定的照度（長壽命性）。爲了回應 此種要求，燈的電極也需要具有良好點弧性及耐消耗性的 材料’也就是說於使陰極的材料，使用使鎢（W)含有氧 化钍（Th〇2)之所 g胃钍鶴（Th〇2-W，thoriated tungsten )(專利文獻1 )。 然而，近年來’根據環境負荷的觀點，逐漸著目於對 於如钍鎢之放射性物質的使用之限制。另一方面，作爲放 電燈’也需要前述電弧安定性與長壽命性。 [先前技術文獻] [專利文獻] [專利文獻1]日本特開昭42-27213號公報 【發明內容】 [發明所欲解決之課題] -5- 201126565 此發明所欲解決之課題係提供抑制钍鎢的使用量之同 時，具有良好電弧安定性與長壽命性的放電燈。 [用以解決課題之手段] 爲了解決前述課題，關於此發明的放電燈，係於放電 容器的內部具有陽極與陰極的構造中，其特徵爲：陰極係 由鎢塡充率是90%以上的钍鎢部，與由連接於此钍鎢部之 純鎢所成的本體部所構成；前述钍鎢部之側面積ST與前述 陰極之側面積S的比例ST/S是0.005以上0.1 5以下。但是， 陰極的側面積S係設爲自陰極之陽極側前端開始的長度爲 陰極最大徑的兩倍爲止的範圍。 進而，特徵爲前述钍鎢部與前述本體部係被擴散接合 (diffusion bonding ) ° [發明的效果] 關於本發明的放電燈係以採用钍鎢部之側面積ST與陰 極之側面積S的面積比ST/S是0.005以上0.1 5以下的陰極， 可減低钍鎢的使用之同時，且以將钍鎢部的鎢塡充率設爲 9 0%以上，使其具有優良電弧安定性與長壽命性者。 進而，關於本發明的放電燈係藉由擴散接合钍鎢部與 本體部，可使钍鎢部所含有的氧化钍（Th〇2)幾乎不還原 而接合本體部。又，在擴散接合中，因爲可利用低於鎢的 熔點之溫度來進行接合，故可維持钍鎢部及本體部的組織 ，除了不影響陰極性能之外，還有在接合後也可切削加工 -6- 201126565 的優點。 【實施方式】 圖1係揭示關於本發明的放電燈之實施例。圖雖然在 說明的便利性上針對放電容器1的發光部2僅揭示內部構造 ’但是’封止部3並未揭示內部構造。 放電燈係整體由石英玻璃製的放電容器1所成，由該 略球狀的發光部2與連續於其兩端而形成之封止部3所構成 。於發光部2的內部，陽極4與陰極5以延伸於放電容器1的 管軸方向之方式配置，兩電極的前端經由數毫米的間隙而 對向配置。又，於發光部2的內部空間係封入有發光物質 或發光用氣體。例如，在身爲液晶或半導體的曝光裝置之 光源的高壓水銀燈之狀況中，作爲水銀（Hg )及緩衝氣 體，封入氙（Xe)氣體或氬（Ar)氣體。又，在身爲放 映機之光源的氙燈之狀況中，封入有氙氣。針對高壓水銀 燈舉出一例時，水銀的封入量是1〜70mg/cm3，氙氣的封 入量是0.05〜〇.5MPa。陽極4係例如藉由鎢含有率是99.9 重量％以上的純鎢來形成整體。關於陰極5係於後詳述。 於此種構造的放電燈中，例如20kV的高電壓被施加 於電極之間時，於電極之間產生絕緣破壞，形成放電電弧 而燈會點燈。在高壓水銀燈之狀況中，以包含波長3 6 5 nm 的i線及波長43 5nm的g線之光爲主，放射線光譜的光’在 氤燈之狀況中’放射從波長300nm至波長110〇11111之連續光 譜的光。 201126565 圖2係圖1所示之放電燈之陰極5的放大圖，特別表示 於軸方向切斷之剖面構造。 陰極5係由從純鎢所成之本體部6，與設置於此本體部 6之陽極側前端的钍鎢部7來構成整體。 本體部6係由鎢含有率99.9重量％以上的純鎢所成，朝 向陽極側前端而漸漸變細之略圓錐台形狀的錐部61，與連 接於此錐部61後端之略圓柱形狀的體部62—體形成。 钍鎢部7係以鎢（W )作爲主成分，作爲射極（易電 子放射性材料），含有氧化钍（Th02 )，亦即，钍鎢（ Th02-W )。具體來說，氧化钍的含有率是2重量％。又， 钍鎢部7的形狀係整體爲略圓錐台形狀，圓錐台的前端面 係對向配置於陽極4的前端之同時，圓錐台的後端面係與 本體部6之錐部6 1的前端面擴散接合。又，钍鎢部7的側面 係具有如持續於本體部6之錐部6 1的側面傾斜之相同的傾 斜，藉由本體部6的錐部61與钍鎢部7，作爲整體，構成陰 極前端的圓錐台形狀。 在此，對於陰極5而存在钍鎢部7的區域係形成放電電 弧的區域或在其附近，直接受到電弧所致之加熱的影響之 區域。爲此’在燈點燈中，包含於钍鎢部7的氧化钍係被 還原成钍原子，擴散於钍鎢部7的內部或外表面，移動至 更前端方向。爲此，钍鎢部7存在的區域即使在陰極整體 中’僅限定於前端的一區域，也可經常將钍良好地供給給 陰極5的前端。結果，可實現可縮小工作係數之同時，具 有優良點弧性與耐消耗性者。 -8- 201126565 又，在燈點燈中，因爲高溫而包含於钍鎢部7的钍也 會蒸發。但是，钍係在電弧中游離成钍離子（Th+)，因 爲本身的極性而被拉引至陰極方向。結果，因爲钍會重複 電弧中之蒸發、钍離子的游離、陰極5的回歸之循環，故 亦可抑制钍的消耗。 另一方面，在先前技術中所說明之陰極5的狀況中， 钍從陰極5的前端以外之區域也會蒸發，故發生多數尙未 前進至電弧中的钍，前述游離並不能太過期待。然後，钍 附著於放電容器1的內壁時會產生白濁，結果，遮蔽了放 射光，導致照度降低而成爲短壽命的原因。本發明係將钍 鎢部7的存在區域限定於陰極5的前端部份之同時，進而， 藉由後述之實驗，利用以規定相對於陰極整體之側面積的 比例，降低無助於前述循環之钍的蒸發。 進而，如前述般，從陰極5蒸發之钍係成爲钍離子而 再次回歸陰極5。但是，陰極5的溫度過於上升時，钍原子 會在放電空間中，附著於溫度較低之放電容器1的內表面 ，與身爲構成放電容器1之材料的矽石（Si02 )產生反應 而產生化合物（白濁）。本發明係爲了解決此種問題，利 用提升包含氧化钍之钍鎢部7的熱傳導性，抑制陰極前端 的過剩溫度上升者。 具體來說，钍鎢部7係鎢塡充率90%以上。尤其，於 輸入電力値是lkW以上的放電燈中，除了前述白濁的發生 ，從耐較高之熱負荷的觀點來看亦有提升熱傳導率的必要 。再者，嚴密上來說钍鎢部7也包含氧化钍，故不僅鎢的 201126565 熱傳導率，也須考慮氧化钍的熱傳導率，但是，氧化钍的 熱傳導率係相較於鎢單體的熱傳導率非常小，故可將鎢塡 充率作爲钍鎢部7之熱傳導性的指標。本案發明的特徵爲 钍鎢部7的鎢塡充率是90%以上，根據熱傳導率較高也稱 爲「高熱傳導钍鎢」。本案發明係利用不僅陰極5之社鎢 部7的比例（在側面積的比例），更規定钍鎢部7的鎢塡充 率，而可達成電弧安定性與長壽命性者。所以，假設即使 僅在陰極5的前端部設置钍鎢之構造已經存在，如果是鎢 塡充率較低者，也無法發揮所希望之熱傳導特性，結果， 可能產生來自陰極前端之過剩钍的蒸發，與放電容器1之 白濁的問題。 在此，鎢的塡充率P係以「P = a ( 1-x ) /19.3」表示。 將構成钍鎢部7之钍鎢的密度（g/cm3 )設爲a，將相對於 氧化钍之钍的重量比設爲X，將鎢的密度（g/cm3)設爲 19.3。a ( 1-x )係每钍鎢的lcm3中鎢所佔之質量，將其除 以鎢的密度19.3 ( g/cm3 )的塡充率P係代表钍鎢中鎢所佔 之體積的比例。如上述般，因爲钍鎢之熱傳導幾乎都藉由 鎢，故鎢所占之體積的比例，亦即，塡充率P越大，钍鎢 的熱傳導性越好》 接著，針對關於本發明之放電燈的陰極5之製造方法 ，說明其一例。 首先，本體部6係利用切削圓柱形狀之鎢的側部，形 成錐部61。另一方面，钍鎢部7係將射極粉末（氧化钍的 粉末）與鎢粉末的混合粉末放入金屬模，施壓而產生一次 -10- 201126565 成形體，並燒結此一次成形體。此時，爲了提升鎢的填$ 率，對於燒結材施加熱加工。具體來說，將被加熱成高溫 的燒結材，以槌子進行型锻（swage )。然後，於鎢塡充 率成爲90 %以上的狀態中，切削此燒結體而使其成爲所胃 望之形狀（例如，圓錐形狀）。 接著，接合本體部6與钍鎢部7。首先，重疊貼合本體 部6之錐部61的前端面與成爲钍鎢部7的後端面，一邊從本 體部6的下面與钍鎢部7的上面加壓，一邊通電加熱。具體 來說，將接合溫度於絕對溫度（K)中設爲材料的熔點之 50〜60%程度，將加壓力設爲數十Pa程度之真空中的接合 溫度之材料的屈服應力（yield stress )之20〜40%程度。 並將此狀態保持到〇·2〜0.3mm程度的縮小量爲止而使其擴 散接合。 所謂「擴散接合」係在面上重疊貼合金屬彼此，在未 滿熔點的固相狀態下不會產生塑性變形之程度加熱•加壓 ，使接合部的原子擴散的固相接合法。 在擴散接合中，加熱溫度是2000 °C程度，如熔融接合 般，不需要加熱至鎢的熔點（約3 4 0 0 °C )爲止，故钍鎢部 7所含有之氧化钍（Th〇2 )幾乎不會被還原。進而，因爲 可維持本體部6及钍鎢部7的組織，故不會對陰極性能造成 不好影響。進而，因爲陰極5的組織不會改變，在本體部6 與钍鎢部7的接合後也可切削加工。 在此，關於陰極5，針對本體部6與钍鎢部7已被擴散 接合之狀況，可利用確認兩者的接合面未熔融及鎢的結晶 -11 - 201126565 粒成長而接合來判斷。具體來說，利用顯微鏡等放大本體 部6與钍鎢部7的接合面，如果存在有超過本體部6與钍鎢 部7的連接處而成長之結晶粒的話，則可判斷兩者是被擴 散接合者。 圖3係揭示關於本發明的放電燈之陰極構造，與圖1不 同之構造。具體來說，圖1所示之陰極5係圓錐台形狀之社 鎢部7的後端面（底面）與由純鎢所構成之本體部6的前端 面幾近以相同徑接合，但是，本實施例是钍鎢部70由圓柱 形狀的體部710與前端部720所構成者，钍鎢部70的體部 7 10嵌合於本體部60的凹部630。再者，钍鎢部70的前端係 如圖所示，作爲圓錐形狀亦可，作爲圓錐台形狀亦可。 接著，針對本發明的效果加以說明。 [實驗例1] 針對圖1所示之構造的本發明相關之放電燈，使钍鎢 部之側面積ST與陰極之側面積S的面積比例ST/S變化而測 定照度維持率。又，作爲比較用燈，使用陰極整體由钍鎢 所構成之放電燈，相同地測定照度維持率。照度維持率係 使燈連續點燈，對於初始照度，測定出降低至50%爲止的 壽命時間。再者，實驗所使用的燈係僅改變相對於陰極的 钍鎢部之體積者，陰極的整體形狀與體積相同。又，陰極 以外的構造也全部相同。 實驗的結果，钍鎢部之側面積ST與陰極之側面積S的 面積比ST/S超過0· 15時，則壽命與比較用燈幾近相同。另 -12- 201126565 —方面，钍鎢部之側面積ST與陰極之側面積S的面積比 ST/S是0.1 5以下時，則可得到關於本發明的放電燈之壽命 比比較用燈長的結果。 進而，比例ST/S小於0.005時，則電弧會極端地不安 定。此理由因钍較少。 結果，可確認於钍鎢部之側面積51與陰極之側面積S 的面積比ST/S在0.005〜0.15的範圍中，至少有比先前的放 電燈在壽命特性及電弧安定性上更有效果。 在此，關於本發明的規定，本質上，如钍鎢部的側面 積與陰極的側面積，可利用側面的面積來評估。但是，伴 隨點燈時間的經過，钍鎢部的前端形狀會變形而使側面與 前端面的邊際不明，故關於本發明之钍鎢部的側面積，也 包含前端面積。 再者，前述雖然是針對氙燈進行實驗者，但是，也針 對高壓水銀燈進行相同的實驗時，關於高壓水銀燈，在钍 鎢部之側面積ST與陰極之側面積S的面積比ST/S是0.005〜 0.1 5時，相較於先前的放電燈在壽命改善效果及電弧安定 性上也可確認相同效果，亦即，陰極整體相較於钍鎢的燈 在壽命改善效果及電弧安定性上可確認相同效果。 再者，關於先前的放電燈，以僅在短時間點燈之新的 放電燈，與長時間點燈後之末期品的放電燈爲對象，使用 X光能量分散式分析儀（energy dispersive X-ray analyzer )，分別觀察陰極表面的钍濃度。結果，可確認後者的放 電燈在陰極之體部徑的兩倍程度之長度爲止，钍濃度會減 -13- 201126565 少，亦即，钍蒸發之狀況於兩倍以上的長度中，钍濃度幾 乎與新的放電燈沒有不同。據此，陰極之钍的蒸發係可確 認在陰極體部徑的兩倍爲止之區域中發生。亦即，也代表 關於面積比ST/S，陰極之側面積S應將陰極之體部徑的兩 倍爲止之長度設爲限度。 【圖式簡單說明】 [圖1 ]揭示放電燈之構成的說明用剖面圖。 [圖2]於軸方向切斷放電燈之陰極的放大剖面圖。 [圖3]於軸方向切斷放電燈之陰極的放大剖面圖。 【主要元件符號說明】 1 :放電容器 2 :發光部 3 :封止部 4 :陽極 5 :陰極 6 :本體部 61 :錐部 62 :體部 7 :钍鎢部 -14-201126565 VI. Description of the Invention: TECHNICAL FIELD OF THE INVENTION The present invention relates to a discharge lamp. In particular, it relates to a discharge lamp that uses yttrium (Th) as an emitter at the cathode. [Prior Art] Conventionally, in a liquid crystal or semiconductor exposure apparatus, a high-pressure mercury lamp was used as a light source, and a xenon lamp was used as a light source of a projector. These discharge lamps are required to stabilize the arc in the lighting (arc stability) and to maintain a certain illuminance (long life) for a long time. In order to respond to this requirement, the electrode of the lamp also needs a material with good arcing and wear resistance. That is to say, the material of the cathode is used to make tungsten (W) contain yttrium oxide (Th〇2). Th〇2-W, specialized tungsten (Patent Document 1). However, in recent years, depending on the environmental load, the restrictions on the use of radioactive materials such as strontium tungsten have been gradually focused. On the other hand, the aforementioned arc stability and long life are also required as the discharge lamp. [Prior Art] [Patent Document 1] [Patent Document 1] JP-A-42-27213 SUMMARY OF THE INVENTION [Problems to be Solved by the Invention] -5- 201126565 The problem to be solved by the present invention is to provide suppression. A discharge lamp with good arc stability and long life while using tungsten. [Means for Solving the Problem] In order to solve the above-described problems, the discharge lamp of the present invention is characterized in that the cathode has a structure in which an anode and a cathode are provided inside the discharge vessel, and the cathode is made of a tungsten ruthenium ratio of 90% or more. The tantalum tungsten portion is formed of a main body portion made of pure tungsten connected to the tantalum tungsten portion, and the ratio ST/S of the side area ST of the tantalum tungsten portion to the cathode side area S is 0.005 or more and 0.15 or less. However, the side area S of the cathode is a range from the tip end of the anode side of the cathode to twice the maximum diameter of the cathode. Further, it is characterized in that the tungsten-tungsten portion and the main body portion are diffusion-bonded. [Effect of the invention] The discharge lamp of the present invention has an area of the side area ST of the tantalum-tungsten portion and the area S of the side of the cathode. The cathode having a ratio of 0.005 or more and 0.15 or less in ST/S can reduce the use of tantalum tungsten and the tungsten crucible content of the tantalum tungsten portion is set to 90% or more, thereby providing excellent arc stability and long life. Sex. Further, in the discharge lamp of the present invention, by the diffusion bonding of the tantalum tungsten portion and the main body portion, the tantalum oxide (Th〇2) contained in the tantalum tungsten portion can be joined to the main body portion with almost no reduction. Moreover, in the diffusion bonding, since the bonding can be performed at a temperature lower than the melting point of tungsten, the structure of the tungsten-tungsten portion and the body portion can be maintained, and the bonding can be processed after bonding without affecting the cathode performance. -6- 201126565 Advantages. [Embodiment] Fig. 1 is a view showing an embodiment of a discharge lamp relating to the present invention. Although the illustration shows only the internal structure of the light-emitting portion 2 of the discharge vessel 1 for convenience of description, the "sealing portion 3" does not reveal the internal structure. The entire discharge lamp system is made of a discharge vessel 1 made of quartz glass, and is composed of the slightly spherical light-emitting portion 2 and a sealing portion 3 formed continuously at both ends. Inside the light-emitting portion 2, the anode 4 and the cathode 5 are arranged to extend in the tube axis direction of the discharge vessel 1, and the tips of the two electrodes are opposed to each other via a gap of several millimeters. Further, a light-emitting substance or a light-emitting gas is sealed in the internal space of the light-emitting portion 2. For example, in the case of a high-pressure mercury lamp which is a light source of a liquid crystal or semiconductor exposure apparatus, xenon (Xe) gas or argon (Ar) gas is enclosed as mercury (Hg) and a buffer gas. Further, in the case of a xenon lamp which is a light source of the projector, helium gas is sealed. When an example is given to the high-pressure mercury lamp, the amount of mercury enclosed is 1 to 70 mg/cm3, and the amount of helium gas enclosed is 0.05 to 55 MPa. The anode 4 is formed integrally, for example, by pure tungsten having a tungsten content of 99.9% by weight or more. The cathode 5 will be described in detail later. In the discharge lamp of such a configuration, for example, when a high voltage of 20 kV is applied between the electrodes, dielectric breakdown occurs between the electrodes to form a discharge arc and the lamp is turned on. In the case of a high-pressure mercury lamp, light mainly comprising an i-line having a wavelength of 3 6 5 nm and a g-line having a wavelength of 43 5 nm, and the light of the radiation spectrum is emitted in a state of a xenon lamp from a wavelength of 300 nm to a wavelength of 110 〇 11111. Continuous spectrum of light. 201126565 Fig. 2 is an enlarged view of the cathode 5 of the discharge lamp shown in Fig. 1, and particularly shows a cross-sectional structure cut in the axial direction. The cathode 5 is integrally formed of a main body portion 6 made of pure tungsten and a tantalum tungsten portion 7 provided at the front end of the anode portion of the main body portion 6. The main body portion 6 is made of pure tungsten having a tungsten content of 99.9% by weight or more, and a tapered portion 61 having a substantially truncated cone shape gradually tapered toward the tip end of the anode side, and a substantially cylindrical shape connected to the rear end of the tapered portion 61 The body 62 is formed in a body. The tantalum tungsten portion 7 contains tungsten (W) as a main component and an emitter (e-electron radioactive material) containing ruthenium oxide (Th02), that is, tantalum tungsten (Th02-W). Specifically, the content of cerium oxide is 2% by weight. Further, the shape of the tantalum tungsten portion 7 is a substantially truncated cone shape, and the front end surface of the truncated cone is disposed opposite to the front end of the anode 4, and the rear end surface of the truncated cone is attached to the front end of the tapered portion 61 of the main body portion 6. Surface diffusion bonding. Further, the side surface of the tantalum tungsten portion 7 has the same inclination as the side surface of the tapered portion 61 of the main body portion 6 is inclined, and the tapered portion 61 of the main body portion 6 and the tantalum tungsten portion 7 as a whole constitute the cathode front end. The shape of the truncated cone. Here, the region where the tungsten-tellurium portion 7 is present in the cathode 5 is a region where the discharge arc is formed or is in the vicinity thereof, and is directly affected by the heating by the arc. For this reason, in the lamp lighting, the yttrium oxide contained in the tantalum tungsten portion 7 is reduced to a ruthenium atom, diffused to the inside or the outer surface of the tantalum tungsten portion 7, and moved to a more front end direction. For this reason, the region where the tantalum tungsten portion 7 exists can be satisfactorily supplied to the tip end of the cathode 5 even if it is limited to only one region of the front end in the entire cathode. As a result, it is possible to achieve an excellent arcing and wear resistance while reducing the working coefficient. -8- 201126565 Further, in the lamp lighting, the flaws included in the tantalum tungsten portion 7 are also evaporated due to the high temperature. However, the lanthanide is freed into cerium ions (Th+) in the arc and is drawn to the cathode direction due to its polarity. As a result, since helium repeats the cycle of evaporation in the arc, the release of helium ions, and the return of the cathode 5, the consumption of helium can also be suppressed. On the other hand, in the case of the cathode 5 described in the prior art, ytterbium evaporates from the region other than the tip end of the cathode 5, so that most of the ruthenium which has not advanced into the arc occurs, and the above-described liberation cannot be expected too much. Then, when 钍 adheres to the inner wall of the discharge vessel 1, white turbidity occurs, and as a result, the emitted light is blocked, resulting in a decrease in illuminance and a short life. In the present invention, the existence region of the tantalum tungsten portion 7 is limited to the front end portion of the cathode 5, and further, by the experiment described later, the ratio of the area to the side of the cathode as a whole is reduced, which does not contribute to the aforementioned cycle. Evaporation of cockroaches. Further, as described above, the ruthenium which evaporates from the cathode 5 becomes ruthenium ions and returns to the cathode 5 again. However, when the temperature of the cathode 5 rises too much, the germanium atoms adhere to the inner surface of the discharge vessel 1 having a lower temperature in the discharge space, and react with the vermiculite (SiO 2 ) which is a material constituting the discharge vessel 1 to generate a reaction. Compound (white turbid). In order to solve such a problem, the present invention has been made to improve the thermal conductivity of the tantalum tungsten portion 7 containing ruthenium oxide and to suppress an excessive temperature rise at the cathode tip end. Specifically, the tantalum tungsten portion 7 has a tungsten germanium charge ratio of 90% or more. In particular, in a discharge lamp having an input power 値 of lkW or more, in addition to the occurrence of the above-mentioned white turbidity, it is necessary to increase the thermal conductivity from the viewpoint of a high heat load resistance. Furthermore, strictly speaking, the tantalum tungsten portion 7 also contains niobium oxide. Therefore, not only the thermal conductivity of tungsten 201126565 but also the thermal conductivity of niobium oxide, the thermal conductivity of niobium oxide is very high compared to the thermal conductivity of tungsten monomer. Since it is small, the tungsten ruthenium charge rate can be used as an index of the thermal conductivity of the tantalum tungsten portion 7. The feature of the present invention is that the tungsten germanium charge ratio of the tantalum tungsten portion 7 is 90% or more, and is also referred to as "high heat conduction tantalum tungsten" depending on the high thermal conductivity. In the present invention, not only the ratio of the tungsten portion 7 of the cathode 5 (the ratio of the side area) but also the tungsten enthalpy charge rate of the tantalum tungsten portion 7 is determined, and the arc stability and long life can be achieved. Therefore, it is assumed that even if only the structure in which the tantalum tungsten is provided at the front end portion of the cathode 5 already exists, if the tungsten crucible is low, the desired heat conduction characteristics cannot be exhibited, and as a result, evaporation of excess helium from the front end of the cathode may occur. , the problem of white turbidity with the discharge vessel 1. Here, the charge rate P of tungsten is expressed by "P = a ( 1-x ) / 19.3". The density (g/cm3) of the tantalum tungsten constituting the tantalum tungsten portion 7 was a, the weight ratio with respect to the tantalum oxide was X, and the density (g/cm3) of tungsten was 19.3. a ( 1-x ) is the mass of tungsten in 1 cm of tungsten per inch, and the ratio of the tungsten to the density of tungsten of 19.3 (g/cm 3 ) represents the ratio of the volume occupied by tungsten in the tungsten. As described above, since the heat conduction of tantalum tungsten is almost entirely by tungsten, the ratio of the volume occupied by tungsten, that is, the larger the charge ratio P, the better the thermal conductivity of tantalum tungsten. Next, for the discharge according to the present invention An example of the method of manufacturing the cathode 5 of the lamp will be described. First, the body portion 6 is formed by cutting the side portion of the cylindrical tungsten shape to form the tapered portion 61. On the other hand, in the tantalum tungsten portion 7, a mixed powder of an emitter powder (a powder of cerium oxide) and a tungsten powder is placed in a metal mold, and a molded body of -10-201126565 is produced by pressing, and the primary molded body is sintered. At this time, in order to increase the filling rate of tungsten, hot working is applied to the sintered material. Specifically, a sintered material which is heated to a high temperature is swaged with a tweezers. Then, in a state where the tungsten ruthenium charge ratio is 90% or more, the sintered body is cut to have a desired shape (for example, a conical shape). Next, the body portion 6 and the tantalum tungsten portion 7 are joined. First, the front end surface of the tapered portion 61 of the overlapping main body portion 6 and the rear end surface of the tantalum tungsten portion 7 are heated while being pressurized from the lower surface of the body portion 6 and the upper surface of the tantalum tungsten portion 7. Specifically, the bonding temperature is set to 50 to 60% of the melting point of the material in the absolute temperature (K), and the yield stress of the material in the vacuum in the vacuum of the tens of Pa is set to the yield stress. 20 to 40% degree. This state is maintained until it is reduced by about 2 to 0.3 mm. The "diffusion bonding" is a solid phase bonding method in which a metal is bonded to each other on the surface and heated and pressurized to cause diffusion of atoms in the joint portion without causing plastic deformation in a solid phase state lower than the melting point. In the diffusion bonding, the heating temperature is about 2000 ° C, and as in the case of fusion bonding, it is not necessary to heat to the melting point of tungsten (about 340 ° C), so the yttrium oxide contained in the tantalum tungsten portion 7 (Th〇2) ) will hardly be restored. Further, since the structure of the main body portion 6 and the tantalum tungsten portion 7 can be maintained, the cathode performance is not adversely affected. Further, since the structure of the cathode 5 does not change, it can be cut after the joining of the main body portion 6 and the tantalum tungsten portion 7. Here, regarding the cathode 5, the state in which the main body portion 6 and the tantalum tungsten portion 7 have been diffusion-bonded can be determined by bonding the joint surfaces of the tungsten and the crystal growth of the tungsten -11 - 201126565. Specifically, by amplifying the joint surface of the main body portion 6 and the tantalum tungsten portion 7 by a microscope or the like, if there are crystal grains growing beyond the junction between the main body portion 6 and the tantalum tungsten portion 7, it can be judged that the two are diffused. Joiner. Fig. 3 is a view showing the configuration of a cathode of a discharge lamp of the present invention, which is different from that of Fig. 1. Specifically, the rear end surface (bottom surface) of the cathode 5 portion of the cathode 5-shaped truncated cone shape shown in FIG. 1 is joined to the front end surface of the main body portion 6 made of pure tungsten in the same diameter, but this embodiment is For example, the tantalum tungsten portion 70 is composed of a cylindrical body portion 710 and a front end portion 720, and the body portion 7 10 of the tantalum tungsten portion 70 is fitted to the concave portion 630 of the body portion 60. Further, the front end of the tantalum tungsten portion 70 may have a conical shape as shown in the figure, and may have a truncated cone shape. Next, the effects of the present invention will be described. [Experimental Example 1] With respect to the discharge lamp of the present invention having the structure shown in Fig. 1, the illuminance maintenance ratio was measured by changing the area ratio ST/S of the side area ST of the tantalum tungsten portion and the side area S of the cathode. Further, as a comparative lamp, a discharge lamp composed of neodymium tungsten was used as a whole, and the illuminance maintenance ratio was measured in the same manner. The illuminance maintenance rate was such that the lamp was continuously lit, and the life time until the reduction to 50% was measured for the initial illuminance. Furthermore, the lamp used in the experiment only changed the volume of the tantalum tungsten portion relative to the cathode, and the overall shape and volume of the cathode were the same. Further, the structures other than the cathode are all the same. As a result of the experiment, when the area ratio ST/S of the side area ST of the tantalum tungsten portion and the side area S of the cathode exceeded 0·15, the life was almost the same as that of the comparative lamp. In addition, when the area ratio ST/S of the side area ST of the tantalum tungsten portion and the side area S of the cathode is 0.15 or less, the life of the discharge lamp according to the present invention can be obtained longer than that of the comparative lamp. result. Further, when the ratio ST/S is less than 0.005, the arc is extremely unstable. There are fewer reasons for this. As a result, it was confirmed that the area ratio ST/S of the side area 51 of the tantalum tungsten portion and the side area S of the cathode was in the range of 0.005 to 0.15, which was at least more effective in life characteristics and arc stability than the previous discharge lamp. . Here, regarding the specification of the present invention, in essence, the side surface area of the tantalum tungsten portion and the side area of the cathode can be evaluated by the area of the side surface. However, with the passage of the lighting time, the shape of the tip end of the tantalum tungsten portion is deformed and the margin between the side surface and the front end surface is unknown. Therefore, the side area of the tantalum tungsten portion of the present invention also includes the front end area. In addition, although the experiment was performed on the xenon lamp, the same experiment was carried out on the high pressure mercury lamp. In the high pressure mercury lamp, the area ratio ST/S of the side area ST of the tantalum tungsten portion and the cathode side area S was 0.005. When the temperature is ~ 0.1 5 , the same effect can be confirmed in terms of the life improvement effect and the arc stability of the conventional discharge lamp, that is, the cathode as a whole can be confirmed in terms of the life improvement effect and the arc stability of the tungsten lamp. The same effect. Furthermore, regarding the previous discharge lamp, a new discharge lamp that is only turned on for a short period of time, and a discharge lamp of the last product after a long time of lighting are used, and an X-ray energy dispersive analyzer (energy dispersive X- Ray analyzer ), respectively, to observe the erbium concentration on the cathode surface. As a result, it was confirmed that the discharge lamp of the latter was less than twice the length of the body diameter of the cathode, and the enthalpy concentration was decreased by -13 to 201126565, that is, the state of enthalpy evaporation was more than twice the length, and the cerium concentration was almost It is no different from the new discharge lamp. Accordingly, the evaporation of the crucible of the cathode can be confirmed in the region of the cathode body portion twice. That is, it is also representative that the area ratio S/S is such that the side area S of the cathode should be twice the length of the body diameter of the cathode. BRIEF DESCRIPTION OF THE DRAWINGS [Fig. 1] A cross-sectional view for explaining the configuration of a discharge lamp. Fig. 2 is an enlarged cross-sectional view showing the cathode of the discharge lamp cut in the axial direction. Fig. 3 is an enlarged cross-sectional view showing the cathode of the discharge lamp cut in the axial direction. [Explanation of main component symbols] 1 : discharge vessel 2 : light-emitting portion 3 : sealing portion 4 : anode 5 : cathode 6 : body portion 61 : taper portion 62 : body portion 7 : tantalum tungsten portion - 14-